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Full text of "Alexander von Humboldt"

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176                           LIFE OF
Thus this time-measuring instrument became as im-
portant for the geologist as the lead is to the mariner.
Both reveal unseen depths ; the variations in the pen-
dulum oscillations even showed whether cavities, or
dense basalt masses, exist in the depth of the earth.
When it was at last possible to ascertain the physi-
cal form of the earth by means of the moon also,
science achieved a great triumph, and Laplace was
justified in saying that an astronomer, without leaving
his observatory, was not only able to determine the
form and size of the earth, but its distance from the
sun and moon, results which could, however, not have
been achieved without long and laborious expeditions
to the most distant portions of both hemispheres. It
is a fact that the form of a planet exercises a consider-
able influence over the movements of other bodies,
especially on the never distant moons., and therefore
the form of the earth can be ascertained by an accii-
rate knowledge of the moon's movements. And what
measurements and pendulums could not establish, was
ascertained by these observations of the irregularities
of the moon's movements, as not only the flattening
of the earth was ascertained by it, but also the proof
acquired that the strata of the earth increase in den-
sity from the surface towards the centre, and thus, as
Humboldt says, the knowledge of exterior forms jus-
tifies conclusions on the inner constitution of a body.
The actual form of tha earth, depending on the in-
equalities of the hardened surface, is to the regular
mathematically precise form as the uneven surface of
an agitated sea to the same surface when calm.
But the earth was not only measured, it was also
weighed, by means of pendulum and lead. If with
these simple instruments the mean density of the
earth could be determined (which is much greater
than pure water, "being 5*44?) its mean weight could
also be ascertained. Even naturalists have advanced
hypotheses on the interior of the earth, whose bulk
must increase in density the nearer it approaches the
centre; these Humboldt rejects, partly as unfounded, ,